4 - LectureNotes2: substance.

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Lecture Notes 2: Physical Equilibria – Phase Diagrams There are number of graphical means to help to understand the relationships between the different phases of a particular substance. The first thing we need to do when looking at the transitions different phases is to establish some definitions. There are three particular phases between which we will be examining discrete phase changes. These are solids, liquids, and gases. Each transition has a particular name. We can view them graphically below along with sign for the changes in enthalpy and entropy associated with each. For example, the transition form liquid to gas is called vaporization. Vaporization is endothermic (uphill in energy), and this transition leads to an increase in entropy. The reverse transition of a gas going to a liquid is condensation. Since condensation is simply the reverse of vaporization, the changes in enthalpy and entropy will be exactly the same, but opposite in sign. So ΔH condensation = ‐ΔH vaporization . Typically, you will only find tabulated values for the endothermic transitions. It would be redundant to list values for the exothermic transitions.
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Let’s take a deeper look at the thermodynamics. In particular, we can examine the relationship between the enthalpy and the temperature during phase transitions. Remember, heat can be tricky. When there is no chemistry or phase transitions, then energy flowing into a system in the form of heat will lead to a temperature change. However, when there is chemistry or a phase transition, then energy will flow in and the temperature can stay constant. Why doesn’t the temperature go up? The energy coming in results in higher potential energy not higher kinetic energy. What is the higher potential energy? Breaking up the IMF between the molecules. This can be easily seen in a heating curve that plots the temperature of a system as a function of the heat flow into the system. Initially the system is a solid, then it has a melting transition, then it is a liquid, then has a vaporization transition, and then it is a gas. This can be seen in the diagram on the left that shows the heating curve for water. Initially, the system is solid water. As the heat flows in, the temperature of the ice increases. The slope of this line is the heat capacity of solid
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4 - LectureNotes2: substance.

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